1. Field of the Invention
This invention relates to a control unit for in-cylinder injection internal combustion engine which can select, according to an engine operation state, a compression stroke injection mode for mainly injecting fuel at a compression stroke and an intake stroke injection mode for mainly injecting fuel at an intake stroke; and, in particular, to a control unit for in-cylinder injection internal combustion engine which can optimally correct an intake amount.
2. Background Art
A throttle valve is installed in an intake passage of an engine such as an internal combustion engine. There has been developed a configuration of an intake system in which a bypass passage bypassing the throttle-valve-installed portion is formed such that its both end portions communicate with the intake passage, and the bypass passage is equipped with a bypass valve.
Commonly employed as such a bypass valve is, for example, a bypass valve for idle speed control which is used for adjusting the intake amount so that the engine can maintain a predetermined idling rotational speed.
Independently of a throttle valve whose opening degree is adjusted in response to a driver's operation for stepping on an accelerator pedal, the opening degree of the idle speed control bypass valve is appropriately adjusted, while feeding back the engine rotational speed, in order to keep the engine rotational speed at a predetermined idle rotational speed, and independently of the driver's accelerator pedal operation, the intake amount is adjusted so as to maintain the idle rotational speed.
Also, there has been developed a configuration of an intake system in which, separately from the bypass passage for idle speed control and the bypass valve, a bypass passage for bypassing the throttle-valve-installed portion of the intake passage is formed so as to be used for controlling the air/fuel ratio of the air/fuel mixture supplied to a combustion chamber not only upon idling, and this bypass passage is equipped with a bypass valve [which is referred to as air bypass valve (ABV)].
Such an air bypass valve can control not only idle speed but also other engine operation states by adjusting the intake amount independently of the throttle valve that is adjusted upon the driver's accelerator pedal operation.
In any case, the opening degree of these bypass valve is appropriately controlled according to the engine operation state or the like, independently of the throttle valve that is adjusted by the driver's accelerator pedal operation.
Also, when the idle speed is to be controlled by such a bypass valve, a target engine rotational speed is set according to cooling water temperature, air conditioner state, shift lever position (in the case of automatic transmission), or the like, and while an actual engine rotational speed is being detected, the opening degree of the bypass valve is adjusted so that the actual engine rotational speed coincides with the target engine rotational speed.
When the bypass valve is used for the purpose other than idle speed control, by contrast, an air amount (intake correction amount) to be taken in by the bypass valve is set according to the engine operation state, and the opening of the bypass valve is adjusted to such a degree that this air amount is attained. In this case, a relationship between the engine operation state and its corresponding intake correction amount or valve opening degree may be mapped, for example, and the intake correction amount or valve opening degree is set while the engine operation state is caused to correspond to this relationship (map).
Further, there is a technique in which a motor-driven throttle valve is provided such that the throttle valve is driven by a motor, while a target opening degree of the throttle valve is set on the basis of various correction coefficients set according to the accelerator opening degree and operation state, thus allowing intake adjustment to be effected without using a bypass valve or the like.
Meanwhile, recently under development are lean-burn engines which perform a burn operation in a fuel-lean air/fuel ratio state. Widely used in such a lean-burn engine is a multi point injection (MPI) type engine in which intake ports of individual cylinders are provided with respective fuel injection valves, since it can control the fuel injection amount in each cylinder and thus has a good controllability. In a recently-developed in-cylinder injection engine (in-cylinder injection internal combustion engine) in which a fuel injection valve is disposed so as to directly inject fuel into a combustion chamber within a cylinder, an ultra lean burn operation, in which a burn operation is performed in a further fuel-lean air/fuel ratio state, can be effected.
Namely, since fuel can be injected into the combustion chamber in such an in-cylinder injection engine at any time regardless of opening and closing of the intake valve, various operations, including the ultra lean burn operation, can be performed as explained in the following.
For example, a fuel injection mode based on a compression stroke (referred to as compression stroke injection mode) can be set. In this compression stroke injection mode, stable burning can be realized in an ultra lean air/fuel ratio state due to stratified combustion utilizing a stratified intake flow formed within the cylinder. Namely, since the injected fuel can be concentrated in the vicinity of a spark plug; while only the vicinity of the spark plug attains a stable ignition performance as an air/fuel ratio state with good ignitability (i.e., state at a stoichiometric air/fuel ratio or an air/fuel ratio in which fuel is somewhat richer than that in the stoichiometric air/fuel ratio), an ultra-lean air/fuel ratio state is attained as a whole so as to enable driving while greatly saving fuel consumption.
Of course, a fuel injection mode based on an intake stroke (referred to as intake stroke fuel injection mode) can be set. In this intake stroke fuel injection mode, while the air/fuel ratio state of the whole combustion chamber is being homogenized by premixing the fuel, operation can be performed so as to realize stable ignition and secure flame propagation, thereby attaining a sufficiently high output. Adopted as this intake stroke injection mode are a stoichiometric mode for attaining a higher output while adjusting the air/fuel ratio to the vicinity of a stoichiometric air/fuel ratio and a lean mode for setting the air/fuel ratio leaner than the stoichiometric air/fuel ratio whereby the fuel consumption can be saved. Also, in view of the case where a high output is temporarily required upon sudden acceleration or the like, adopted is an enriched mode in which the air/fuel ratio is made richer than the stoichiometric air/fuel ratio.
Such an in-cylinder injection engine is operated while individual operation modes such as the above-mentioned compression stroke injection mode (compressed lean mode or later lean mode), intake stroke injection stoichiometric mode (stoichiometric mode), intake stroke injection lean mode (intake lean mode or earlier leanmode), intake stroke injection enriched mode (enriched mode), and the like are appropriately selected. These operation modes are considered to be selected according to engine rotational speed and engine load.
Namely, the compressed lean mode is selected in a region where both engine rotational speed and engine load are low; whereas, as the engine rotational speed or engine load increases therefrom, the intake lean mode, stoichiometric mode, and enriched mode are successively selected in response to the magnitude of increase.
Since the amount of stepping on the accelerator pedal substantially corresponds to the engine load; when the compressed lean mode is selected, the accelerator pedal stepping amount is small, whereby the opening degree of the throttle valve is small. In the compressed lean mode where operation is effected in an ultra lean state with a very high air/fuel ratio, by contrast, unless a sufficient amount of intake air is provided, the stratified flow is weakened, thereby making it difficult to effect stable burning. Accordingly, when the throttle valve opening is small and the intake air amount is restricted, it becomes difficult to effect operation in the compressed lean mode.
Therefore, in the in-cylinder injection engine, as mentioned above, a bypass passage (air bypass passage) bypassing the throttle valve is formed, and the intake amount correction is performed so as to compensate for the air amount restricted by the throttle valve, while controlling a valve (air bypass valve) attached to the air bypass passage.
Meanwhile, in such an in-cylinder injection engine, among the operation modes, the air/fuel ratio to be controlled varies, and the fuel injection stroke changes, whereby the amount of intake required for obtaining the same torque differs among the operation modes, and the intake vacuum for obtaining the same torque differs among the operation modes as well.
Since the intake vacuum for obtaining the same torque differs among the operation modes, when the relationship between the engine operation state and the intake correction amount or valve opening degree is uniformly set as mentioned above, for example, an optimal intake correction amount cannot be set for each operation mode, thereby deteriorating drivability.
In view of the problems mentioned above, it is an object of the present invention to provide a control unit for an in-cylinder injection combustion engine, which can set an appropriate intake correction amount for each operation mode in the in-cylinder injection internal combustion engine in order to improve the drivability in the in-cylinder injection internal combustion engine.